Document feeders which use endless conveyor belts to advance original documents across the exposure platen of an electrophotographic copier are well known in the art. In feeders of the type shown in U.S. Pat. No. 4,023,791 to Hori et al and U.S. Pat. No. 3,844,552 to Bleau et al, a plurality of transversely spaced belts advance the document to a registration edge of the platen where fingers of a controllable registration gate interdigitate with the belts to arrest the document. While the use of spaced belts and an interdigitating registration gate in this manner ensures reliable document registration, the belts also appear along the margins of copies made from smaller-than-normal originals. Document feeders such as that shown in U.S. Pat. No. 3,747,918 to Margulis et al which use a single wide belt provided with spaced longitudinal sheet-engaging ridges share this drawback, owing to the clear line between the ridges and the flat part of the belt. Still other feeders such as disclosed in U.S Pat. No. 4,093,372 to Guenther which use a single flat belt to obviate this problem do so only at the cost of reliable document registration. Since the registration gate cannot interdigitate with the belt, the leading sheet edge will tend to work its way between the feed belt and the registration gate and beyond the registration edge of the platen.
Bleau U.S. Pat. No. 3,910,570 discloses a document feeder in which a plurality of axially spaced pressure rollers interdigitating with the upwardly extending fingers of a retractable exit gate are urged downwardly against the lower run of the feed belt to ensure that the leading edge of the document remains in contact with the exposure platen. May U.S. Pat. No. 4,247,095 discloses a similar arrangement in which, rather than separate axially spaced pressure rollers, a single pressure roller with reduced-diameter portions opposite corresponding raised portions of a registration edge is used to urge the leading edge of the document against the exposure platen. While these arrangements may reduce or eliminate the problem of background clutter referred to above, they also introduce an undesirable amount of complexity into the overall system.
Other problems arise from the use of a registration gate to align documents along the registration edge of a platen. Since the documents are advanced across the exposure platen at fairly rapid speeds, 75 inches per second not being an uncommon figure, their sudden stop at the registration edge requires large decelerative forces that can damage the leading edge of the document being fed. In addition, owing to the high-speed impact of the document against the registration gate, the document tends to rebound from the edge, resulting in misregistration. While these undesirable effects can be reduced somewhat by reducing the document speed, such a reduction in speed also reduces the copy rate obtainable using the document feeder
Still other problems associated with automatic document feeders of the prior art relate to the separating of individual sheets from a stack and the advancing of them to the exposure station. Automatic feeders must be able to advance documents through, or to a registered position within, the copier exposure station in an extremely short period of time to maintain a high copy rate, especially if the document must remain stationary in the exposure station for an appreciable portion of the copy cycle while being scanned by a moving-mirror copier optics. One way to increase the effective copy rate, shown in Fackler U.S. Pat. No. 3,556,512, is to move a second document to a ready position, separated from the stack and adjacent the exposure station, while a first document is being scanned. Not only does this reduce the distance that documents to be copied must be transported during the scanning return stroke, but it reduces additional delays due to slippage, since the document to be copied is already in the nip formed by opposing pinch members rather than awaiting separation from a stack by opposing feed and retard members or the like.
Although this disclosed system of feeding a second sheet to a ready, or preregistered, position while a first sheet is being copied permits a higher copy rate, certain problems remain. Since Fackler requires additional feed elements to advance the preregistered document to the exposure station, he introduces a discontinuity of gripping surfaces between the stack and the station. Not only does this obviously increase the complexity of the overall feeder, but it also increases the chances for skewing, slippage or the like.
Caldwell U.S. Pat. No. 4,043,665 discloses a semiautomatic document feeder in which documents inserted manually past a switch sensor are advanced to a preregistration position defined by gates just downstream of a set of registration rollers initially retracted from the platen belt. When a preceding document on the platen has been copied, the registration rollers are moved into engagement with the belt to grip the preregistered sheet, and the registration gates are simultaneously retracted to permit the sheet to be moved onto the platen. While such an arrangement locates preregistered documents in a nip formed by the platen belt itself, it is still a relatively complicated arrangement, owing to the various gates and retractable rollers that must be actuated at the proper time.
Guenther U.S. Pat. No. 4,093,372 discloses a recirculating document feeder in which the bottom sheet of a stack of documents is pre-inserted into the nip formed by a pair of opposing feed rollers which are actuated to advance the sheet to the exposure platen. However, Guenther does not support the stack of documents at a location spaced from the feed rollers, but rather maintains the stack in a shingled (i.e., partially separated, partially overlapping) configuration to avoid the necessity of opposing sheet-feeding and sheet-separating members. Not only is such a configuration easily disturbed, but it imposes an additional limit on the maximum number of sheets that can be contained in the stack while maintaining reliable operation. In addition, since Guenther provides a second, independently driven pair of opposing feed members between the first pair and the exposure platen, he subjects the sheets to a discontinuity in drive force as the sheets enter the second nip.
Suzuki et al U.S. Pat. No. 4,009,957 discloses an electrophotographic copier in which copy sheets are advanced into a preregistration nip formed by stationary, contacting registration rollers so as to buckle the sheet in the nip. The rollers are subsequently driven to feed the preregistered sheets to the transfer station of the copier. Suzuki et al do not disclose such a preregistration arrangement in the context of a document feeder, however, nor do they disclose a system in which sheets are advanced to a preregistration position defined by a roller and a belt.
Hori U.S. Pat. No. 4,231,562 discloses a feeder similar in some respects to that of Caldwell in which sheets are automatically fed from a stack to a preregistration position defined by a gate. The gate is immediately downstream of an initially open nip formed by drive rollers, which are coaxial with a pulley supporting the platen belt, and a set of pressure rollers which are subsequently moved into engagement with the drive rollers to advance the preregistered sheet to the platen. Since, however, documents are not advanced to the drive and pressure rollers while these feed members are in a nip-forming configuration, a feed discontinuity, and thus occasion for sheet damage, is introduced when the nip-forming members are ultimately engaged with each other.
Still another problem associated with document feeders of the prior art involves the detection of the misfeeding of sheets from a stack of documents. It is common in the art, as shown, for example, in Burlew et al U.S. Pat. No. 4,078,087, to test for misfeed by examining a downstream sheet sensor after a predetermined time period has elapsed following the actuation of a sheet-feeding member to advance a sheet from the stack. If the sheet has not reached the sensor within the predetermined time period, a jam is presumed, and a fault declared However, when a stack of documents is first placed in the document tray of the feeder, a substantial period may elapse before the stack has settled into a position assuring adequate contact between the driven feed members and the sheet to be fed. If a time period is selected which is short enough to detect jams before significant damage to documents occurs, that same time period may result in a premature declaration of a fault due to a jam when the document tray is initially loaded.